1. Field of the Invention
This invention relates to wireless communication systems.
2. Description of the Related Art
Communication systems, such as wireless systems, are designed to meet various subscriber demands. Service providers continuously seek ways to improve the overall performance of the communication system. In the past, wireless communication systems have been used for voice communications, but more recent technological developments have allowed high speed data transmission as well. As wireless communications become more and more popular for subscribers to obtain data (i.e., e-mail or information from the internet), communication systems should be capable of a higher throughput and be tightly controlled to maintain a high quality of service. Communication is conducted according to any desired communications standard, such as the Universal Mobile Telecommunications Standard (UMTS) or a CDMA standard.
As is known in the art and shown generally in FIG. 1, a wireless communication system 100 serves a service coverage area that is divided into cells 101 having one or more sectors 102. Base stations 104, 104a are associated with the sectors 102 in at least one cell 101. Adaptive modulation and coding allows selection of an appropriate transport format (e.g., modulation and coding) for the current channel conditions seen by the user. There are two directions of data flow in such systems; communications from the base station 104, 104a to a mobile device 106 are considered to flow in a downlink direction, while communications originating at the mobile device 106 and sent to the base station are considered to flow in an uplink direction. A given base station is considered a serving base station 104a if it is currently transmitting data to the mobile device 106. These base stations, and other base stations 104, 104a that may be chosen by the mobile device 106 as a candidate for acting as a new serving base station, are collectively known as the “active set” of base stations. Thus, the serving base station 104a is considered to be a member of the active set of base stations. Control information, such as information required for power control of transmissions by the mobile device 106, is transmitted by the entire active set of base stations.
The CDMA standard, and specifically CDMA200 Revision D, incorporates several additional air-interface control mechanisms that control transmissions from the mobile device 106 on the uplink to a given base station 104, 104a. The control mechanisms allow the base stations 104 in the system 100 to perform various functions with respect to mobile device 106 transmissions. These functions include schedule grants that schedule the mobile device 106 for data transmission (i.e., direct the mobile device 106 to transmit up to a selected data rate and/or power level), rate control commands that indicate any changes in transmission rates of the mobile devices 106, and acknowledgements that acknowledge successful or failed reception of transmissions from the mobile devices 106.
As is known in the art, the mobile device 106 communicates with just the serving base station 104a when no other base stations are in the active set. When the mobile device 106 starts communicating concurrently with other base stations (i.e., the active base stations 104, including the serving base station as well as additional base stations), the mobile device 106 is considered to be in a handoff situation. A signaling message, such as a Universal Handoff Direction Message (UHDM), is sent to the mobile device 106 by one or more of the base stations 104, 104a to move the mobile device 106 into a handoff state.
As shown in FIG. 1, the mobile device 106 conforming to a CDMA standard, such as the CDMA2000 Revision D standard, receives a scheduling grant from only the serving base station 104a. However, rate control commands are sent from all active set base stations, while acknowledgements may be sent from some or all of the active base stations 104, but always the serving base station 104a. 
Typically, three separate shared downlink control channels at each base station 104, 104a are used to communicate the schedule grants, rate control commands, and acknowledgements from the base stations 104, 104a to the mobile devices 106. Further, a specific sub-channel on each of these channels communicates control information to a given mobile device 106. The identity of these shared downlink control channels and code or time-slot sub-channels must be communicated to the mobile device 106 for each base station in the active set even if the system 100 does not intend to send scheduling, rate control, and/or acknowledgement information from all of the active base stations 104 104a. This is because the mobile device 106 or a controller in the system 100 may change the base station acting as the serving base station 104a at any time.
One possible method to communicate the scheduling, rate control, and/or acknowledgement channel and sub-channel identities is to communicate the channel and sub-channel identities to the mobile device by including this information in the UHDM. This method is enabled by the standard. Setting the identities of the forward control channels of each active base station 104, 104a a priori via the UHDM when the mobile device 106 moves into the handoff state requires each base station 104, 104a communicating with the mobile device 106 to reserve some or all of its capacity on the control channels for communication on the downlink. Further, sending the control channel identity information via a message, such as the UHDM, imposes a substantial cost in down link power in addition to forcing each base station 104, 104a to reserve downlink Walsh codes, bit positions, etc.
For mobile devices 106 in the handoff state, each active set base station 104, 104a may send control information that is the same as or different from control information from other base stations 104, 104a receiving communications from the same mobile device 106. For example, one of the active base stations 104, 104a may send a positive acknowledgement when it has successfully received a transmission from the mobile device 106 while at the same time another active base station 104, 104a may respond with a negative acknowledgement. Similarly, one base station 104, 104a may ask the mobile device 106 to increase its transmission rate while at the same time another base station 104, 104a may ask the mobile device 106 to decrease its transmission rate. The mobile device 106 evaluates the information from the multiple base stations and sets its response (e.g., transmission rate, re-transmissions, etc.) based on this evaluation.
Not all of the base stations 104, 104a, however, that are transmitting over the shared downlink control channels know the quality of their respective downlinks to the mobile device 106. This is because the mobile device 106 typically provides downlink quality reports only for a single base station sector 102 (i.e., the serving sector). As a result, other base station sectors 102 do not have any associated downlink quality reports that would be used to allocate appropriate power levels on their downlink control channels to the mobile device.
There is a desire for a method that allows control information from multiple base stations to be transmitted to a single mobile device without excessively reducing resources available for other traffic between the base stations and the mobile device.